Probes are used to make contact with circuits-under-test. For example, if an electrical engineer wants to observe the signal activity on a circuit board trace, the engineer may select an active, high-impedance probe such as a P7260 from Tektronix, Inc. of Beaverton, Oreg. When the engineer touches the probe's fine tip to the trace, a high input-impedance probe amplifier in the probe tip senses the signal and sends a buffered replica to an oscilloscope for display.
Designing a probe for high-speed microelectronic circuits is very challenging. The ideal probe is easy to connect to the circuit-under-test and has high input-impedance (high resistance, low inductance, and low capacitance). Unfortunately, most common techniques used to make probes easy to connect to the circuit-under-test result in excess inductance and capacitance, thereby spoiling the measurement bandwidth and fidelity of the probe. Furthermore, each time a user touches the probe tip to the trace, the user may approach it from a slightly different angle or exert a slightly different force, resulting in slightly different contact resistance, inductance and capacitance and therefore unrepeatable measurements. Soldering the probe tip to the trace or using a mechanical probing arm may alleviate these problems somewhat, but not entirely. These challenges only get worse as microelectronic circuits get smaller and faster.
Other challenges to probing a signal are that the probe amplifier input voltage range may be smaller than the voltage of the signal-under-test and that the input capacitance of the probe amplifier may be undesirably high. To alleviate these problems, probes use passive attenuators to attenuate the signal-under-test before it is amplified, thereby reducing the voltage applied to the probe amplifier and making the input capacitance seen from the probe tip smaller by a factor equal to the attenuation ratio. Unfortunately, these passive attenuators result in increased expense, complexity, and equivalent noise at the input.
A further challenge is that in some cases the user does not see a true representation of the transmitted signal unless the trace is probed at particular locations. For example, if the trace is a source terminated transmission line, the probed signal appears distorted unless the trace is probed at the very end of the transmission line, or at the immediate input to the receiver. This is due to the fact that on a source terminated transmission line the transmitted signal (the “forward” or “incident” wave) reflects from the far end of the line and propagates back toward the transmitter (the “reverse” or “reflected” wave); however, the sum of the forward and reverse waves (what the probe actually observes) appears distorted everywhere except at the very end of the transmission line. Unfortunately, the input to the receiver is not generally accessible for probing; it may be, for example, contained within a packaged integrated circuit. A related challenge is in verifying the termination quality of a load-terminated transmission line. Here, the object is to verify that all of the data transmitted to a receiver is absorbed, or equivalently that no data is reflected. As such, the user wants to measure the reflected wave without the incident wave, and for the same reason as was discussed above, a conventional probe is insufficient. Similarly, in the case of a bidirectional communications link, if the user wants to measure only signals traveling in a particular direction across the link, a conventional probe is again insufficient because it measures the summation of signals traveling in both directions.
One way to separate forward and reverse waves is to use a directional coupler. A typical directional coupler is a 4-port passive microwave circuit such as the BDCA1-7-33 from Mini-Circuits of Brooklyn, N.Y. If the BDCA1-7-33's port 1 and port 2 are inserted into the conductor of a circuit-under-test, a portion of the forward wave traveling from port 1 to port 2 appears at port 3, and a portion of the reverse wave traveling from port 2 to port 1 appears at port 4. Unfortunately, these types of directional couplers are narrowband and are therefore not suitable for measuring broadband data signals. For example, the BDCA1-7-33 is only suitable for signals between 1.6 GHz and 3.3 GHz. This is insufficient to measure modern broadband serial data signals such as 3.125 Gb/s XAUI (Extended Attachment Unit Interface) which typically has frequency content from DC to well beyond 3.125 GHz.
U.S. Pat. No. 3,934,213 describes other types of directional couplers that use amplifiers to measure and algebraically combine the currents and voltages on a transmission line in order to form representations of the forward and reverse waves. This approach takes advantage of the fact that the voltages and currents in forward waves are in-phase while the voltages and currents in reverse waves are 180 degrees out-of-phase. These directional couplers are broadband in that they operate from DC up to the bandwidth of the technology used. Unfortunately, they require measurements from several points on a transmission line and in some cases even cutting the transmission line to make a series measurement. Thus, these types of directional couplers are not naturally suited to conventional hand-held probing techniques.
What is desired is a high-bandwidth, high-fidelity probe that is easy to connect to microelectronic traces and provides repeatable measurements. It is further desired that the probe have a reasonable input range and low input capacitance while avoiding the expense, complexity, and input noise penalty associated with passive attenuators. It is further desired that the probe have broadband directional sensing capability.